1. Field of the Invention
The present invention relates to a charge coupled device suitably employed in a solid-state imaging device for use in, for instance, a digital camera, and to the production thereof
2. Related Background Art
A charge coupled device (hereinafter referred to as CCD) is a device having a structure in which a multiplicity of metal oxide semiconductor (MOS) structure electrodes are arranged on a surface of a semiconductor, and generally is used as a so-called solid-state imaging device in various types of electronic cameras, facsimiles, etc.
FIG. 2 illustrates a cross-sectional structure of a prior art CCD. A charge transfer region 22 is formed on a P-type silicon substrate 21 in which an impurity is doped, and a gate insulation film 23 is formed on the charge transfer region 22. On the gate insulation film 23, first gate electrodes 24 and second gate electrodes (transfer electrodes) 27 are formed alternately. When a voltage is applied to the gate electrodes, charge stored under each gate electrode is transferred from one electrode to another successively, and the charge is outputted as a voltage at an output section. 28 denotes a potential well.
The following will describe a method for producing the prior art CCD. First, as shown in FIG. 3A, the charge transfer region 22 is formed on the P-type silicon substrate 21, and the gate insulation film 23 is formed on the charge transfer region 22. Subsequently, the first gate electrodes 24 are formed on the gate insulation film 23 by patterning.
Next, as shown in FIG. 3B, the first gate electrodes 24 are oxidized so as to be covered with a silicon oxide film 25. Subsequently, as shown in FIG. 3C, the second electrodes 27 are formed on the gate insulation film 23 so that each is arranged between adjacent first gate electrodes 24. Thus, the first and second gate electrodes 24 and 27 are arranged alternately on the gate insulation film 23, and the silicon oxide film 25 is formed between the gate electrodes, whereby the gate electrodes are insulated electrically from one another by the silicon oxide film 25.
In the case where this method is used, the forming rate of the silicon oxide film 25 decreases with increasing proximity to the gate insulation film 23 since the supply of oxygen decreases. Therefore, as shown in the drawings, on side faces of the first gate electrode 24, the thickness of the silicon oxide film 25 decreases (the silicon oxide film 25 is constricted) with increasing proximity to the gate insulation film 23. The second gate electrode 27 is formed in a state of being in contact with the silicon oxide film 25, and hence, it is formed to have acute-angle edges 32 in its base part on the gate insulation film 23. When a voltage is applied to each gate electrode in the foregoing state, sometimes an electric field is concentrated at the edges 32, thereby causing dielectric breakdown between the first gate electrode 24 and the second gate electrode 27.
To cope with the foregoing problem, conventionally the varying of the conditions for the oxidization of the first gate electrode 24 has been attempted, or alternatively, the increase in the dielectric breakdown voltage between the gate electrodes was attempted by increasing a film thickness t2 (FIG. 3B) of the silicon oxide film 25 on the first gate electrode 24.
However, the foregoing problem has not been solved by any one of the foregoing methods, and particularly the increase in a film thickness t2″ (FIG. 3B) of the silicon oxide film 25 necessarily increases a distance g2 (FIG. 3C) between the gate electrodes, thereby impairing the charge transfer efficiency of the CCD. Furthermore, at the same time, a difference t2′–t2″ between the film thickness t2′ of the silicon oxide film 25 on the side faces of the first gate electrode 24 and the film thickness t2″ thereof in contact with the gate insulation film 23 increases, thereby narrowing the angle of the edges 32 on the gate insulation film 23. As a result, the dielectric breakdown tends to occur more easily.